1. Field of the Invention
Embodiments of the present invention relate, in general, to Broadband Wireless Access (“BWA”) communication systems, and more particularly to methods for controlling sleep modes for real-time services in BWA communication systems so as to reduce terminal power consumption with minimal quality deterioration.
2. Relevant Background
Broadband Wireless Access is emerging as an integral part of the next generation (4G) wireless access infrastructure. BWA is aimed at providing wireless access to data networks with high data rates. One particular BWA technology is being standardized by IEEE 802.16. According to the 802.16-2004 standard, broadband means “having instantaneous bandwidth greater than around 1 MHz and supporting data rates greater than about 1.5 Mbit/s.” From the point of view of connectivity, BWA is an attractive alternative to traditional cable modem, xDSL and T1/E1 connections. At the same time, by taking advantage of the inherent mobility of wireless media, BWA is considered as a candidate technology that enables users, when mobile, to make use of a wide-area network through an access network.
In wireless communication systems, battery power is typically a scarce resource. The batteries of mobile stations have a limited life before they must be recharged. Therefore, a major challenge in operation of wireless communication systems is the efficient use of a limited battery power resource. There are many ways of reducing energy consumption throughout various layers in wireless communication systems. Wireless communication systems primarily occur in the Data-Link layer. Recall that the Open System Interconnection (“OSI”) reference model includes 7 primary layers, with each primary layer capable of possessing sub-layers. The primary layers include a Physical layer, a Data-Link layer, a Network layer, a Transport layer, a Session layer, a Presentation layer and finally an Application layer. Sub-layers of the Data-Link layer in which energy consumption can be reduced includes the Media Access Control (“MAC”) sub-layer and the Logical Link Control sub-layer. It is the OSI Reference Model Data-Link layer that determines who is allowed to access the physical media at any one time. Recall that the Physical layer, the layer next to the Data-Link layer, is level one in the seven level OSI model of computer networking. It is in the Physical layer that services requested by the Data-Link layer are preformed and refers to network hardware, physical cabling or wireless electromagnetic connections.
Wireless communication occurs in the Data-Link layer thus sleep mode control is properly addressed in this layer. Of the Data-Link's sub-layers, the MAC sub-layer sleep mode control is the simplest way to reduce power consumption. The MAC sub-layer acts as an interface between the Logical Link Control sub-layer and the network's Physical layer. The MAC sub-layer is primarily concerned with the control of access to the physical transmission medium (i.e. which of the stations attached to the wire or frequency range has the right to transmit) or low-level media-sharing protocols like Carrier Sense Multiple Access With Collision Detection.
The general idea of sleep mode control is to put the mobile station into a low power consumption mode when it is not involved in any communications. A mobile station goes into sleep mode after negotiating with the base station and periodically wakes up for a short interval and checks whether there is downlink traffic present. Based on the presence or absence of downlink traffic during these checks, the mobile station elects either to return to sleep mode (i.e. no downlink traffic) or go into active mode (downlink traffic present).
FIG. 1 is a timeline of the typical relationship between sleep and listening intervals that occur when a mobile station enters into sleep mode as is known in the prior art. As shown, a mobile station is generally either in active mode 110 or sleep mode 120. Before the mobile station can enter into sleep mode, the mobile station in active mode must send a sleep request to the base station and await the base station's approval 130.
After gaining approval 130 from the base station, the mobile station goes into sleep mode 120 and enters into a sleep interval 140. Shortly thereafter the mobile station wakes and enters a listening interval 150 to listen for traffic addressed to it. When communications traffic is present the mobile station exits sleep mode 120 and reenters active mode 110. When no traffic is present, after listening for the prescribed interval, the mobile station enters into another sleep interval 140. As long as no traffic is presented to the mobile station during the listening interval 150, the mobile station will remain in sleep mode 120.
One standard for access to Broadband Wireless established by the Institute of Electrical and Electronics Engineers (“IEEE”) is referred to as 802.16e. This standard is aimed at filling the gap between fixed wireless local area networks and mobile cellular systems. To increase a mobile station's ability to exist in a standby mode, it proposes the use of sleep mode control. 802.16e allows a mobile station to have multiple connections with differentiated Quality of Service (“QoS”). Sleep mode under 802.16e is based on the concept of power saving classes. Power saving classes are defined as a group of connections that have common demand properties. For each involved mobile station, the base station keeps one or several contexts, each one related to a certain power savings class. A particular power savings class may be repeatedly activated and deactivated. Activation of a certain power savings class results in starting successive sleep/listening cycles associated with each class. Generally there are three power saving classes. Each differs by their parameter sets, procedures of activation/deactivation and policies of mobile station availability for data transmission. Power saving class of type I is typically recommended for non-real-time connections of Best Effort and non-real-time Variable Rate connections. Power saving class of type II is typically recommended for real-time connections including Unsolicited Grant Service and real-time Variable Rate type connections. Power saving class of type III is typically recommended for multicast connections as well as for management operations, for example, Periodic Ranging, DSx operations, Mobile Neighbor Advertisement, and the like. Power saving class of type I enters sleep mode 120 at the frame specified as Start Frame Number for the first sleep interval 140. Sleep intervals 140 are interleaved with listening intervals 150 of fixed duration. Each updated sleep interval 140 thereafter is twice the size comparatively to the previous one, but not greater than a specified final value.
During the sleep interval 140 of a power savings class type I operation, the mobile station is not expected to send or receive any traffic connections that belong to power savings class I. During the listening interval 150, the mobile station is expected to receive all downlink transmissions in the same way as it would in active mode 110.
Sleep intervals 140 associated with power savings class II enters are all of the same length and are interleaved with listening intervals 150, also of a fixed duration. During the sleep interval 140 the mobile station is prevented from receiving or transmitting data. During the listening interval 150 the mobile station can both send and receive any packets as well as acknowledge that it has received them.
Power savings class III enters a determinable sleep interval 140 much like power savings class I and II. The duration of the sleep interval 140 is a determinable period of time. After the expiration of the sleep interval 140 power savings class III operations automatically go back to active mode. FIG. 2 shows a prior art timeline of a typical behavior of a mobile station with two power savings classes according to 801.16e sleep mode standards. Class A 210 contains several best effort and non-real-time variable rate connections. Class B 220 contains a single unsolicited grant service connection. For class B 220 the base station allocates a sequence of listening intervals 230 of a constant duration and a sleep interval 240 of a constant duration. For Class A 210 the base station allocates a sequence of listening intervals 250 of constant duration and doubles that length for the sleep interval 260. The mobile station is considered to be unavailable 270, i.e. powered down, only when there are intersected sleep intervals 240, 260 of class A 210 and class B 220. Similarly, intervals of availability 280 exist when only one or neither of the classes are experiencing a sleep interval 240, 260.
Although sleep mode control has proved to be an effective tool for power resource savings, the savings comes at the price of a deterioration of QoS. A long sleep interval is clearly advantageous from the perspective of power conservation, but such a prolonged interval creates an extended buffering delay. Alternatively, a short sleep interval decreases the buffer delay but results in frequent awakenings to check packet intervals thus expending valuable power resources. A tradeoff exists therefore between a longer sleep interval, which conserves power, and a shorter sleep interval that provides better performance.
One attempt to resolve this dilemma was offered in a paper entitled, “MAC Sleep Mode Control Considering Downlink Traffic Pattern and Mobility,” Proc. IEEE, Vehicular Technology Conference, Vol. 3, pp. 2076-2080, 30 May-1 Jun. 2005. This prior solution proposes a sleep mode interval control algorithm according to MAC states. The MAC states are classified into two modes: sleep mode and awake mode. Two states in sleep mode are suggested, doze and power save. Which states in sleep mode the mobile station enters into is based on the MAC state in awake mode. When the MAC state in awake mode is idle, i.e. no connections for data transmissions exist, the mobile stations can enter a doze state. In doze the mobile station goes to sleep. When the MAC state in awake mode is active, but there has been a relatively long period wherein there has been no traffic, the mobile station can enter a power save state. Power save is of a short time period in comparison to doze. Therefore the sleep mode control method is different for power save and doze states.
While this approach provides the mobile station with a greater degree of flexibility to manage its power consumption, the deterioration of QoS during long sleep intervals remains unaddressed. This problem is especially pronounced when real-time services are considered.